Battery Second-Life Capacity Calculator

Screen retired batteries for their next stationary role

Second-life battery screening is rarely decided by one capacity percentage alone. A pack can leave vehicle duty because its range, power delivery, or warranty margin no longer suits transportation, while still storing useful energy for backup power, home storage, telecom support, demand shifting, or light commercial stationary service. The practical issue is whether the retired battery still has enough usable energy for its proposed duty and whether its recorded history points to substantial degradation. This calculator provides a first-pass estimate from original energy capacity, cycle history, operating temperature, depth of discharge, and age.

The calculator reports estimated remaining usable capacity in kilowatt-hours and as a percentage of initial capacity. It also presents a logistic screening score based on the estimate's distance from 80% of original capacity. Treat that score as a way to rank candidates and compare input scenarios, not as a measured three-year forecast or a guarantee of future battery behavior. Capacity testing, impedance checks, insulation tests, visual inspection, thermal review, and battery-management-system diagnostics remain necessary before installation.

Why battery history inputs affect second-life capacity

This second-life capacity estimator uses five history inputs that are often available from service records, inspection notes, or a BMS log: original energy, full-equivalent cycles, typical depth of discharge, average temperature, and calendar age. They cannot describe every electrochemical process, but together they provide a practical initial picture of the wear a retired pack or module may have experienced.

Initial Capacity (kWh) is the battery's nameplate energy when new or when first commissioned. If you are evaluating a module, enter the module's own energy capacity, not the original energy of the entire vehicle pack. This matters because the result is scaled directly from the starting energy. If the original capacity is overstated, every remaining-capacity estimate will be overstated too.

Completed Cycles should be entered as full-equivalent cycles. That means two 50% swings count roughly as one full cycle, and four 25% swings count as one full cycle. Full-equivalent cycles are more useful than raw charge events because they tie wear to actual energy throughput. In the model used here, cycle damage grows with the square root of cycle count rather than as a perfectly straight line.

Average Depth of Discharge describes the typical fraction of the battery used per cycle. A battery that regularly swings through 90% of its capacity generally accumulates more wear than one that moves through 40% or 50% per cycle, even with a similar cycle count. This matters for a second-life system because its planned duty cycle may use shallower swings to limit later degradation. If the historical value is uncertain, compare several plausible cases rather than relying on one guessed percentage.

Average Temperature is a long-run operating temperature in degrees Celsius. Higher temperature can accelerate battery aging, particularly when warm conditions persist. If only ambient temperature is available, use it cautiously as a proxy. The form accepts non-negative values because this is a simplified capacity-screening model, not a detailed low-temperature performance model. Batteries regularly exposed to freezing conditions need more specific testing.

Age represents calendar aging. A battery can lose capacity over the years even if it is lightly cycled. This is particularly relevant to fleet batteries that spent extended periods parked, on standby, or partially charged in storage. An older battery with a warm history deserves more caution than a similarly cycled battery kept cooler.

The form's prefilled values merely demonstrate how the second-life estimator responds. They are not operating targets or a claim that every reuse candidate has the same history. Replace them with your own pack or module records before using the result for screening.

Battery degradation model used for the retained-capacity estimate

This battery second-life calculator subtracts four modeled effects from an initial retained fraction of one: square-root cycle wear, calendar aging, a temperature adjustment to calendar aging, and a depth-of-discharge cycle term. The script uses the following retained-fraction model:

Here, C₀ is original capacity, N is completed full-equivalent cycles, D is average depth of discharge, T is average temperature, and A is age in years. The script clamps the retained fraction between 0 and 1 so the capacity estimate cannot be negative or exceed the original capacity. Temperature is referenced to 25 °C: temperatures above that baseline add to the model's aging penalty, while temperatures below it reduce that particular term. The model does not imply that cool operation reverses past degradation.

The displayed risk percentage is a logistic transformation of the gap between estimated capacity and 80% of the original capacity, using 5% of original capacity as its scale. It rises as the estimated capacity approaches or falls below that threshold. Because no additional future aging term appears in the equation, it should be read as a proximity-based screening signal rather than a literal prediction of capacity loss over a defined number of years.

Worked second-life estimate using the sample battery history

Suppose a retired EV pack started at 60 kWh, has completed 800 full-equivalent cycles, usually saw 80% depth of discharge, averaged 25 °C, and is 5 years old. At 25 °C, the temperature adjustment is zero relative to this model's baseline. Cycle wear, age, and the depth-of-discharge cycle term produce a retained fraction of about 0.9146. Multiplying by the initial capacity gives an estimated remaining usable capacity of about 54.9 kWh.

For that 60 kWh pack, 80% of initial capacity is 48 kWh. The calculator's proximity-based screening score is roughly 9.2% because the estimated 54.9 kWh remains well above that line. The result panel labels this history as excellent for reuse under its simple scoring rules. That label is not an installation approval; it only indicates that these broad history inputs do not by themselves identify a low-capacity candidate.

When comparing retired batteries, vary one history input at a time. Increasing average temperature by 10 °C or raising depth of discharge from 70% to 90% shows how this specific model changes retained capacity. A scenario sweep is often more useful than treating one estimate as a definitive measurement.

Second-life battery history scenarios in this model

This comparison holds original capacity at 60 kWh and changes the recorded battery history. It does not predict every chemistry or pack design. It illustrates how the calculator's stated degradation model responds to gentler service, the prefilled retired-EV example, and a hotter, more deeply cycled history.

Reading a second-life capacity result responsibly

When the calculator reports remaining capacity, relate the estimated kWh to the stationary application you are considering. A backup system may value usable energy more than vehicle-style acceleration or rapid charging. A battery that no longer fits transportation can therefore remain useful in a lower-stress role. The retained-kWh figure and retained percentage help identify that possibility, but neither measures power capability or safety condition.

The screening score is best used for triage. A low value means the modeled capacity is comfortably above the 80% reference point. A middle value suggests caution and can justify derating, shallower planned cycling, additional monitoring, or direct capacity testing. A high value means the modeled estimate is near or below the reference point and calls for rigorous verification before reuse. It is not a certification, warranty determination, or time-based probability forecast.

Check that the battery estimate moves in the expected direction when you edit the form. More age, more full-equivalent cycles, hotter average operation, and deeper discharge all reduce the retained fraction in this model. If a result seems surprising, first verify the units, the full-equivalent-cycle interpretation, and whether the original capacity belongs to the entire pack or only one module.

Capacity is only one part of second-life suitability. A battery can retain substantial energy yet have power, imbalance, internal-resistance, thermal, or safety-system constraints that make a proposed project unsuitable. Likewise, a modest-capacity battery can be useful for a gentle application with controlled temperature and shallow cycling. Use this calculator near the start of a screening workflow, before detailed testing and system design.

Limits of this battery reuse screening tool

This second-life capacity estimator is intentionally simple. It assumes the five form inputs summarize operating history adequately for an initial screen. It does not distinguish chemistry, charging rate, storage state of charge, pack imbalance, thermal gradients, repair history, or cell-level faults. It also does not project a separate future degradation path; its score only reflects the modeled capacity estimate relative to the 80% reference. Use it to rank options, identify candidates for testing, and discuss reuse strategy—not as the sole basis for warranty, compliance, or safety decisions.

A practical battery-reuse workflow is to screen histories with this calculator, perform direct electrical and safety tests on the most promising units, and then specify a conservative stationary duty cycle. That sequence can focus detailed testing on batteries whose documented history and estimated retained energy justify further evaluation.